Peter Mandelson set up Willbury Limited to handle income from public speaking and books. Photograph: Samir Hussein/Getty Images

Peter Mandelson received £400,000 as a cash loan last year from a company he owns, in a move which could ease his tax burden, accounts filed recently at Companies House reveal [see footnote].

The company, of which he is the sole shareholder, gave the former secretary of state a loan for that amount in the financial year 2013/14 – a move described by a leading tax campaigner as likely to have been motivated by tax avoidance.

Salary payments and dividends from a small business are liable for tax under UK rules, but in the case of a loan to a director – provided a certain minimum rate of interest specified by HMRC is charged – the borrowing is not liable for tax. The official interest rate that applied at the time was 3.25%.

The accounts for Willbury Limited – a company set up by Mandelson two weeks after the 2010 general election to receive income from his book and public speaking engagements – show the company charged £15,211 in interest on the balance, an amount it refers to as the official rate.

The document shows no repayments were made on the loan by Mandelson during the year, nor was any repayment schedule or term of loan set out.

Such a measure is perfectly legal, but it allows those who take advantage of the mechanism to delay when they pay tax on income earned through their company – potentially indefinitely.

Richard Murphy, a chartered accountant and director of Tax Research UK, said Mandelson’s use of loans raised questions.

“How to extract cash from small companies whilst paying as little tax as possible on the way is a massive part of the UK tax avoidance industry,” he said.

“Directors taking loans from companies they own is one way in which this is done, which has been widely condemned in the past when done by footballers and others. It’s just about impossible to think this is motivated by anything but tax avoidance.

“All politicians, including members of the House of Lords, should not only be seen to comply with the spirit of the law on tax but should be required to do so as a condition of holding office.”

When contacted by the Guardian about the arrangement, a spokesperson for Lord Mandelson said the peer and his company paid all relevant taxes.

The Guardian put to Lord Mandelson that a loan of the sort he had taken from his company could serve to potentially delay tax indefinitely, especially given there was no term of repayment specified in the documents. A spokesman for Mandelson did not address the specific point, but instead issued a statement.

It said: “Willbury Ltd is the company that oversees all of Lord Mandelson’s writing, public speaking, broadcasting and personal commercial undertakings. It pays all relevant UK corporate taxes and Lord Mandelson pays all relevant personal taxes.”

Mandelson recently criticised Labour’s tax policies on wealthy individuals. Speaking on Newsnight last Monday about the mansion tax, Mandelson said Ed Miliband’s plan to tax properties worth more than £2m was “crude” and “sort of short-termist” and would “clobber” homeowners.

Mandelson’s loan and property dealings previously came under scrutiny after he was forced to resign from Tony Blair’s cabinet in 1998 when it was revealed he failed to declare a secret property loan from Geoffrey Robertson, a fellow Labour MP and tycoon, who was at the time under investigation by Mandelson’s department.

After a brief return to the cabinet, he was forced to resign a second time in 2001 over his involvement in a passport application of Indian billionaire Srichand Hinduja.

Mandelson’s comments on wealth and tax have long attracted a great deal of attention. During the early years of Blair’s premiership, he famously commented to an audience of tech entrepreneurs that he was “intensely relaxed about people getting filthy rich as long as they pay their taxes”.

Since Labour’s election defeat in 2010, Mandelson’s attention has turned to the private sector. In addition to releasing his autobiography entitled The Third Man – which had paperback sales of around 13,000 – he has made public speaking appearances for companies including Coca-Cola, Lloyds bank and Samruk-Kazyna, Kazakhstan’s sovereign wealth fund, as well as for mining companies operating in Africa.

Separately, Mandelson established a second company, Global Counsel LLP, in partnership with his long-time aide Ben Wegg-Prosser and the advertising giant WPP.

Global Counsel’s clients include multinationals such as BP and commodities giant Glencore.

In 2012, the Guardian revealed that Mandelson was advising Asia Pulp and Paper, which has been accused by environmental groups of destroying large swaths of an Indonesian rainforest, although the company has now given a commitment to protect forests.

• This article was amended on 30 January 2015. An earlier version described the loan as “tax-free”. To clarify: a payment representing 25% of the sum advanced to the director is payable to HMRC if the loan has not been repaid within nine months and a day of the company year end, but this is repayable if the loan is refunded by the director. This article was further amended on 18 February 2015 to clarify that Lord Mandelson’s speaking engagements were not for clients, that Goldman Sachs is not his client and that Asia Pulp and Paper has now given a commitment to protect forests.